PROJECT SUMMARY Salmonella enterica serovars are pathogenic bacteria that cause significant morbidity and mortality in humans worldwide. Gastroenteritis is the most prevalent of the clinical syndromes associated with Salmonella enterica serovars and is caused by nontyphoidal Salmonella enterica serovars such as Salmonella enterica serovar Typhimurium (STm) in immunocompetent individuals. As part of our ongoing studies aimed at elucidating mechanisms by which STm establish infection and avoid immune clearance, we began to investigate the role of inflammatory monocytes (IM, CD11b+ Ly6Chi Ly6G- cells) in STm-induced colitis. IM are innate immune cells that play a key role in immunity and host defense. IM originate from progenitors in bone marrow (BM) and, under certain pathological conditions, can be recruited from BM into peripheral tissues. We previously published that IM purified from tissues of mice infected with STm exhibit both protective and immunosuppressive properties that may influence the outcome of the infection. Furthermore, we recently published that IM provide a niche for STm expansion in the lumen of the inflamed intestine. These published findings from our laboratory established that IM play a key role in the pathogenesis of STm-induced colitis. Most recently, we have found that IM purified from ceca of mice infected with STm produce tumor necrosis factor (TNF)-?, a cytokine that mediates the early pathology in STm infection of the gastrointestinal tract; and express the receptor for interferon (IFN)-?, a cytokine that delays resolution of STm-induced intestinal inflammation. The objective of this application is to define how IM contribute to the pathogenesis of STm- induced colitis, a high impact topic important for understanding the role of IM in immunity and host defense. Our central hypothesis is that IM contribute both directly and indirectly to the pathogenesis of STm-induced colitis. To test different aspects of our central hypothesis and accomplish our objective, we will 1) identify the cecal cell type(s) that produce the monocyte chemoattractant CCL2 during STm infection and determine whether Ccl2 expression by myeloid cells is required for the accumulation of IM in ceca of mice infected with STm, 2) define how IM are exploited to promote STm expansion in the lumen of the inflamed intestine, and 3) determine whether Tnfa or Ifngr1 expression by IM contributes to the severity of the pathology of STm-induced colitis. Conceptual advances resulting from the proposed research are expected to provide new, fundamental insights into host interactions with bacterial pathogens and thus will have a strong and sustained influence on the field.